Cellular Proliferation and Cancer
The disruption of external or internal regulation of cellular growth can lead to uncontrolled cellular proliferation and in cancer, tumor formation. This loss of cellular growth control can occur at many levels and, indeed, does occur at multiple levels in most tumors. Under these circumstances, although tumor cells can no longer control their own proliferation, such cells still must use the same basic cellular machinery employed by normal cells to drive their growth and replication.
Mitosis and Spindle Formation
In a process known as mitosis, cancer cells, like all mammalian cells, multiply through replication and segregation of the original chromosomes. Following DNA replication in the S phase, the cells progress in the G2 phase. During the G2 phase, cells continue to increase in mass and prepare for mitosis. If chromosome damage is present in the G2 phase, the affected cell responds by activating the G2 phase checkpoint, which prevents progression into mitosis. In the absence of DNA damage or following repair of damage, the G2 phase cells then enter the M phase in which the identical pairs of chromosomes are separated and transported to opposite ends of the cell. The cell then undergoes division into two identical daughter cells.
In a process known as spindle formation, the cell utilizes the mitotic spindle apparatus to separate and pull apart the chromosomes. This apparatus, in part, consists of a network of microtubules that form during the first stage of mitosis. Microtubules are hollow tubes that are formed by the assembly of tubulin heterodimers from alpha- and beta-tubulin. The assembly of tubulin into microtubules is a dynamic process with tubulin molecules being constantly added and subtracted from each end.
Vinca Compounds as Inhibitors of Mitosis and Cellular Proliferation
In general, vinca compounds are known to be inhibitors of mitosis and cellular proliferation. In particular, the antiproliferative activity of the vinca alkaloid class of drugs has been shown to be due to their ability to bind tubulin. Assembly of tubulin into microtubules is essential for mitosis and the binding of the vincas to tubulin leads to cell cycle arrest in M phase and subsequently to apoptosis. For example, at low concentrations, these compounds interfere with the dynamics of microtubule formation. At higher concentrations, they cause microtubule disassembly, and at still higher concentrations, the formation of tubulin paracrystals.
Moreover, the anti-cancer activity of vinca alkaloids is generally believed to result from a disruption of microtubules resulting in mitotic arrest. However, cytotoxicity of vinca alkaloids also has been demonstrated in non-mitotic cells. Considering the role of microtubules in many cellular processes, the cytotoxic action of vinca alkaloids may involve contributions from inhibition of non-mitotic microtubule-dependent processes.
Cytotoxicity may also be a consequence of changes in membrane structure resulting from the partitioning of vinca alkaloids into the lipid bilayer. Studies with another tubulin binding compound, taxol, have shown that cell cycle arrest was not a precondition for apoptosis by agents of this type. Therefore, the anti-cancer activity of vinca alkaloids may be the result from disruption of a number of distinct microtubule-dependent and possibly microtubule-independent processes.
The assembly of tubulin into microtubules is a complex process involving dynamic instability (i.e. the switching between periods of slow growth and rapid shortening at both ends of the microtubule), and treadmilling (i.e. the addition of tubulin to one end of the microtubule occurring at the same rate as loss of tubulin from the other). Low concentrations of vinca alkaloids have been shown to bind to the ends of the microtubules and suppress both microtubule instability and treadmilling during the metaphase stage of mitosis. For example, vinca alkaloids have been shown to stabilize microtubule plus ends and destabilize microtubule minus ends. Although the spindle is retained under these conditions, there is frequently abnormal alignment of condensed chromosomes. At higher concentrations of vinca alkaloids, the spindle is not present and the chromosome distribution resembles that of prometaphase cells. At both low and high concentrations of vincas, mitotic arrest results from activation of metaphase-anaphase checkpoint. The molecular basis of this checkpoint is a negative signal sent from the kinetochore of chromosomes that are not attached to microtubules. This signal prevents the activation of pathways that result in the initiation of anaphase events.
Although there is a common binding site for the vinca alkaloids on tubulin, the members of this class do behave differently. The relative overall affinities for β-tubulin binding are vincristine >vinblastine >vinorelbine >vinflunine, but there is no significant difference in the affinity of all four drugs for tubulin heterodimers. The discrepancy has primarily been explained by differences in the affinities of vinca-bound heterodimers for spiral polymers and the binding of drug to unliganded polymers. For example, tubulin spirals induced by vinflunine are significantly smaller than those induced by vinorelbine.
In addition, vinca alkaloids also differ in their effects on microtubule dynamics. Vinflunine and vinorelbine suppress dynamic instability through: slowing the microtubule growth rate, increasing the mean duration of a growth event and reducing the duration of shortening. In contrast, vinblastine reduces the rate of shortening and increases the percentage of time the microtubules spend in the attenuated state. Vinblastine, vinorelbine, and vinflunine all suppress treadmilling, with vinblastine displaying the greatest potency.
In Vivo Properties
The vinca derivatives fall into the general class of cytotoxic anti-cancer agents and, as such, suffer from the same problem as all cytotoxics—i.e., toxicity. Vincristine and vinblastine are neurotoxic. Vinorelbine, which is structurally very similar to vinblastine and vincristine and is only slightly less potent, is less neurotoxic. This change in toxicity cannot be explained by examination of the binding affinity of these compounds for tubulin alone. It has been postulated to arise from an increase in sensitivity to changes in microtubule dynamics in tumor cells and, as described above, these compounds have been shown to have subtly different effects. It could also arise from changes in cellular uptake of the drug. Vinflunine is not very potent in vitro yet is active in vivo, and this has been attributed to its superior cellular uptake. There are also quite significant differences in the profile of efficacy of vinca alkaloids. Vincristine has found wide use in the treatment of hematologic malignancies including leukemias and lymphomas. It is also widely used in pediatric solid tumors and, in the past, in small cell lung cancer. Vinblastine is an important component of the combination regimen that is curative for testicular cancer. Vinorelbine is quite different and has found use mainly in breast cancer and non-small cell lung cancer.
There remains a need for novel vinca derivatives with improved pharmacological and therapeutic properties, improved processes for the preparations of such vinca derivative compounds, corresponding pharmaceutical compositions, and methods of use.
The present invention is directed to achieving these objectives.